One of the major difficulties of bone tissue engineering lies in mimicking the organo-mineral composites of natural bone. This is desirable since successful integration of an orthopedic implant into neighboring bony tissue would be characterized by the implant surface becoming part of the dynamic bone remodeling process. Implant surface features such as roughness and surface chemistry have been shown to play a critical role in osteoblastic differentiation and proliferation. Replicating natural bone is a challenge given its highly complex composition and organization. Natural bone can be thought of as an organo-mineral composite with seven levels of hierarchical organization, of which the basic building block is the mineralized collagen fibril. Each Type I collagen fibril is formed from self-assembly of three polypeptide chains into a triple helix. The principle mineral in bone, hydroxyapatite (Ca10(PO4)6OH2), is believed to grow out of fibril channel gaps to form arrays of flat, nano-crystalline plates with their crystallographic c-axes aligned with the fibril long axes. It would be desirable to approach hard tissue engineering by patterning it after this biotemplating concept.
Titanium is valued for use in orthopedic surgery implants as a result of its excellent biocompatibility and mechanical properties, including high strength to weight ratio, toughness, and processibility. Titanium's surface is covered with a surface oxide layer that serves to give titanium its biocompatibility in vivo and makes titanium a relatively bioinert surface that does not elicit an immune or inflammatory response. It has been shown in the literature that coating this oxide surface with hydroxyapatite improves bone response and increases implant interfacial strength. It would be desirable to create a biomimetic hydroxyapatite organo-mineral material that would similarly elicit a favorable bone response.
Techniques of tissue engineering employing biocompatible scaffolds provide viable alternatives to prosthetic materials currently used in prosthetic and reconstructive surgery (e.g. craniomaxillofacial surgery). These materials also hold promise in the formation of tissue or organ equivalents to replace diseased, defective, or injured tissues. In addition to their use in the biocompatible scaffolds, biodegradable materials may be used for controlled release of therapeutic materials (e.g. genetic material, cells, hormones, drugs, or pro-drugs) into a predetermined area. Most polymers used today to create these scaffolds, such as poly(lactic acid), polyorthoesters, and polyanhydrides, are difficult to mold and hydrophobic, resulting in, among other things, poor cell attachment and poor integration into the site where the tissue engineered material is utilized.